1. Field of the Invention
This invention relates to an output circuit, specifically, an output circuit having slew rate adjustment circuits.
2. Description of Related Art
Output circuits generally have a power MOS transistor, and control the switching of the power source to the load. In the output circuits, a sudden transition of the switching element causes the voltage variation due to the inductance components on the line or the like. Accordingly, the output circuits usually have a slew rate adjustment circuit which adjusts the slew rates during the turn-on and turn-off transitions. The slew rate is defined as the speed of the voltage variation during the transition in the switching element. For example, the slew rate during the turn-off transition is set relatively low so that the excessive high switching speed would not degrade the stability of the circuit operation.
Japanese Unexamined Patent Application Publication No. H11-346147 discloses an output circuit which has a slew rate adjustment circuit. FIG. 3 shows the arrangement of the output circuit described in the reference above. At the time when an input pulse signal Vi ramps up to High level from Low level, the P-channel transistor Q21 is turned on. This allows the gate terminal of the output transistor (hereafter referred to as output MOS) 21 to be charged by the constant current source CS21. The gate voltage of the output MOS21 rises and then the output MOS21 is turned on. The rise speed of the voltage is determined by the current value of the constant current source CS21. That is, in the output circuit 20, the slew rate during the turn-on transition is set according to the current value of the constant current source CS21.
On the other hand, at the time when the input pulse signal Vi ramps down to Low level from High level, the N-channel transistor Q22 is turned on. This allows the constant current source CS22 to discharge the electrical charge having been accumulated at the output MOS21. The gate voltage of the output MOS21 falls and then the output MOS21 is turned off. The fall speed of the voltage is determined by the current value of the constant current source CS22. That is, in the output circuit 20, the slew rate during the turn-off transition is set according to the current value of the constant current source CS22.
By the way, the output circuits usually have a protection circuit which avoids the persistent excessive current on the output MOS21. When such a protection circuit detects the excessive current state, the input pulse signal Vi is forced to drop to Low level and the output MOS21 is turned off. When an excessive current occurs at the output MOS21, the output MOS21 is under the stress corresponding to the energy that would be generated during the time between the beginning of the occurrence of the excessive current and the completion of the turn-off of the output MOS21. In this case, if the slew rate during the turn-off transition is low enough not to degrade the stability of the circuit operation, the forward safe operation area (SOA) of the output MOS21 may be exceeded, which causes a danger of the destruction of the output MOS21.
In the circuit 20, also when the output MOS21 is forced to be turned off by the protection circuit, the MOS21 is turned off at a slew rate for the turn-off transition which is determined based on the current value of the second constant current source CS22. In view of the protection for the output MOS21, a higher slew rate during the turn-off transition is preferable. But, as described above, so high a slow rate causes the problem of degradation in the stability of the circuit operation when the rated current is passed through the output MOS21. In other words, in the conventional output circuit 20, there is a tradeoff between ensuring the protection of the output MOS21 and improving the stability of the circuit operation. Thus, the effective protection of the output MOS21 and the adequate improvement of the stability of the circuit operation have not been compatible.